The present invention relates generally to an apparatus for measuring a distance to an object by means of triangulation using infrared light which is emitted from the light emitting device to the object.
Distance measuring apparatuses have been used in variable applications such as cameras, surveying instruments, position sensors, robots, object sensors, collision preventing apparatuses, automatic doors or the like.
In an exemplary distance measuring device, infrared light is emitted from a light emitting diode (LED) toward an object and is then reflected from the object to a position sensing device (PSD) which can generate two output signals corresponding to positive and negative amounts of deviation of infrared light received therein with respect to a center thereof. Exemplarily, the LED is driven to turn on and off at a constant frequency so that infrared light blinking at a constant frequency can be emitted therefrom. Such infrared light emitted from the LED passes through a first lens toward an object. The infrared light reflected from the object is then received in the PSD through a second lens which is spaced from the first lens. When the infrared light is received in the PSD, the PSD generates two output signals which correspond respectively to positive and negative amounts of deviation of the light received in the PSD with respect to the center thereof. According to a principle of triangulation, a distance to the object can be obtained with a relatively high accuracy by obtaining a ratio of one of the output signals of the PSD to the other.
However, when a distance to an object is longer than about ten meters, output signals of the PSD becomes extremely small, resulting in an extreme deterioration of precision of measurement. Also, when external lights are too intensive, it becomes impossible to measure a distance with a sufficient accuracy even if though a distance to an object is shorter than about ten meters.
In order to solve these problems, it can be considered to increase an output of the LED. This, however, causes the life of the LED to be shortened.
As another method for solving the above-described problems, Japanese Unexamined Utility Model Publication No. 64-15907, published on Jan. 26, 1990, discloses means for measuring a distance to an object, in which a pair of amplifier are provided for amplifying two output signals of the PSD, respectively, and a signal pick-up circuit including a pair of filters and detectors is provided for picking up a signal component having a frequency which corresponds to a frequency of blinking of infrared light from each of the output signals so as to identify the infrared light received in the PSD from external lights which ar also received in the PSD. Gain controlling means are further provided for automatically changing gains of the amplifiers in such a manner that when, for example, a signal component picked up from one of the output signals is smaller in magnitude than, a predetermined level, it causes the gains of the amplifier to be increased. This publication, however, does not disclose a specific internal construction of the gain controlling means.